(1) Field of the Invention
This invention relates to a porous glass film, preferably in the form of a hollow filament, in which the pore size is controllable, and to a method for its preparation.
(2) Description of the Prior Art
In recent years, much research on porous membranes usable as ultrafiltration membranes and the like has been carried out. Since mainly high molecular weight materials are used for such porous membranes, those membranes have defects with respect to chemical resistance, mechanical strength, heat resistance and the like. Moreover, it has been very difficult to control the pore size of those membranes. Although Nuclepore (a trade name), which is made of a polycarbonate and developed by General Electric Co. in the U.S.A., is known as a porous membrane in which the pore size is controllable, it has the above-mentioned problems with respect to chemical resistance and the like because it consists of a high molecular weight material. Therefore, if a porous membrane having a desired pore diameter is prepared by the use of a ceramic material of excellent heat resistance and chemical resistance, very extensive applications can be expected for it.
Japanese Patent Publication No. 44580/1978 discloses a method for preparing porous glass in a hollow filamentous form by heat-treating sodium borosilicate glass spun into a hollow filament to separate the glass into a soft phase which is easy to elute with a hot acid solution and a hard phase which is difficult to elute with the solution, and then subjecting the glass filament to an acid extraction to make the glass porous. Furthermore, it discloses a method for increasing the pore diameter through sodium hydroxide-treatment. However, when such a porous glass filament is used as a porous membrane such as an ultrafiltration membrane or the like, sufficiently satisfactory results cannot be obtained with respect to permeability, uniformity of surface pores, control of pore size, smoothness of the surface and the like.
The present inventors have researched the preparation of porous glass in a hollow filamentous form and have found in the course of their research that the glass layers scarcely separated into the two phases were formed on the surfaces of the hollow filament during a spinning process or heat-treatment process for phase separation and that the layers became various barriers because they acted as a defense layer in acid extraction, caused a decrease in the permeability of the resulting porous glass and made control of the pore diameter on the surface difficult. The inventors further devoted themselves to the research and, as a result, have found that the above-mentioned layers not easily phase-separated could be removed by plasma etching in the presence of atomci fluorine. Thus, the inventors have accomplished a method for preparation of an improved porous glass film.
Incidentally, the term "porous glass film" used herein means a film such as membranes composed of porous glass having flat or curved structures, which is specifically exemplified by a hollow porous glass filament having a film-like wall.